Gate valves of the sliding gate and segment type have been used extensively for many years, and from the beginning, the design engineer has faced this problem. At either end of the stroke of the pair of members, they must be expanded in thickness by stopping one member and pushing or pulling on the other to cause it to climb the wedging surface of the halted member, thus sealing against the valve seals. Between extremities, on the other hand, it is necessary to minimize the thickness of the sliding members so that they will not bind against the valve seats and jamb the gate and segment in a half-and-half position.
Prior art solutions to this problem fall into two general categories, springs and centralizers. The former is exemplified in the U.S. patent to Dunbar and Sanders, issued in 1960 as U.S. Pat. No. 2,954,960. This patent discloses the use of an ox yoke spring over a central pin in one member and having its ends tucked under a pair of opposed pins protruding from the other member. The spring forces the two members together so long as no opposed axial forces are exerted on the members. At the end of a stroke, when the floating segment member is halted by contact with a fixed stop member, an axial force exerted on the gate member through the control stem causes it to climb up the now fixed segment member, thus expanding the assembly and causing it to seal against the opposed valve seats. It should be noted that with this type of assembly, the force applied to the gate member must first overcome the spring force.
The centralizer type of structure is exemplified in U.S. Pat. No. 2,583,512, issued to Laurent in 1952. Even a summary description of this structure would be too lengthy to set forth here, but it will be noted that the structure is complicated and includes a pair of fixed guides and a mechanical interlock between gate and segment. The latter includes a guide shoe which, during any sliding movement of the gate and segment, is constrained to travel in contact with both fixed guides; in such position, the elements linked to the guide shoe force the gate and segment to stay in a central position in which they are pressed together with both pairs of inclined surfaces in tight contact.